This invention relates to a method of determining the relative air content in a product which in the form of an air-containing liquid or cream flows through a conduit. The method includes transferring a sample of the flowing product from the conduit to a sampling chamber which can be isolated from the conduit and which has a variable volume, effecting a predetermined variation of the volume of the chamber and hence of the sample after the chamber has been isolated from the conduit and measuring the pressure in the chamber resulting from the volume variation.
In the industrial production of ice-cream, souffle, pudding and many other products, air or other gas is mixed with or whipped into a liquid initial material having a more or less viscous or stiff consistency, after which the product is pumped through a conduit, usually at a certain positive pressure, to a dispensing station at which it is discharged into containers, for example portion packages for retail sale. For the sake of convenience the term "air" is used herein to designate any gas which may be added to a fluent material for modifying its consistency.
The magnitude of the relative air content, which in the production of ice-cream is also known as the swelling degree, is defined as the ratio between the volume of the quantity of air at atmospheric pressure and the volume of the liquid phase. It is generally of importance for the price of the final product as well as for its quality. In the law, maximum values of the air content of foodstuffs and similar edible products are often laid down, and it is then in the interest of the producer that the quantity of air can be kept as closely as possible below the value permitted.
Normally, the admixture of air is controlled by manually adjusting a reduction valve through which compressed air is introduced into the initial material. The adjustments are effected on the basis of the operator's experience combined with intermittent measurements carried out on the dispensed product. The measurements are generally carried out by withdrawing a sample having a certain volume, for example half a liter, in the dispensing station and weighing the sample. The air content, measured in per cent, can then be determined by comparing the specific gravities of the sample and of the initial material. This measuring method is not very accurate and is rather time-consuming, so that in practice the measurements are carried out only at comparatively long intervals, during which the parameters relevant to the air content, including the composition and temperature of the initial material, may however vary considerably, which further reduces the accuracy of the method. In addition, a comparatively long time elapses before a change in the supply pressure of the air gets any influence on the final product discharged at the dispensing station, and the withdrawal of each sample causes a certain waste which may be considerably larger than the volume of the sample.
It is known to control the air content in ice-cream by supplying compressed air to the liquid mix through a nozzle which operates at the critical pressure ratio so that the flow rate of air through the nozzle depends solely on the pressure upstream of the nozzle. This method requires a comparatively expensive equipment and it requires a high quality of the pumps for transporting the cream. Furthermore, an adjustment of the equipment is necessary every time the output of the ice-cream freezer is varied.
From the specification of U.S. Pat. No. 2,141,977 there is known an apparatus for determining the air content in for example ice-cream. The apparatus is designed as a cock connected to a conduit through which the ice-cream flows. The rotatable plug of the cock contains a sampling chamber through which the cream flows in one position of the plug and which is isolated from the conduit when the plug is rotated 90.degree.. A piston may be pressed into the sampling chamber, whereby the volume of the chamber is reduced and at the same time the pressure rises in the trapped ice-cream sample. The final pressure in the sample depends on the relative air content of the cream.